1. Field of the Invention
The present invention is directed to methods for preparing substantially pure crystalline SAPO-31 silicoaluminophosphate molecular sieve ("SAPO-31"). In particular, the present invention is directed to methods for preparing SAPO-31 which methods employ specified crystallization media with specified pHs. When such crystallization media are employed, the resulting SAPO-31 is substantially pure.
2. Background of the Invention
Crystalline silicoaluminophosphate molecular sieves are well known in the art and are particularly useful as molecular sieve adsorbents and in hydrocarbon conversion processes. In particular, European Patent Application Publication No. 209 227, among others, describes dewaxing processes utilizing catalysts containing a crystalline silicoaluminophosphate molecular sieve and a hydrogenation metal such as platinum or palladium. In this reference, it is disclosed that one of the preferred silicoaluminophosphate molecular sieves for use in the therein described processes is SAPO-31. The preparation and characterization of crystalline silicoaluminophosphate molecular sieves are described in detail by Lok et al., U.S. Pat. No. 4,440,871, the disclosure of which is incorporated herein by reference. Such crystalline silicoaluminophosphate molecular sieves have threedimensional microporous crystal framework structures of PO.sub.2, AlO.sub.2 and SiO.sub.2 tetrahedral units. A number of different crystalline silicoaluminophosphate molecular sieves have been synthesized, each with a characteristic crystal structure as evidenced by unique X-ray diffraction patterns for each structure. Crystalline silicoaluminophosphate molecular sieves are conventionally termed "SAPO" and each of the different SAPO crystal structures are referred to by separate numbers. For example, Lok et al., U.S. Pat. No. 4,440,871, describes the synthesis of numerous SAPO crystalline molecular sieves including, SAPO-5, SAPO-11, SAPO-16, SAPO-17, SAPO-20, SAPO-31, SAPO-34, SAPO-37, SAPO-40, SAPO-41, SAPO-42, SAPO-44 and the like.
It is also known in the art that in the synthesis of SAPO-31 to obtain a product of any significant purity the final reaction mixture was required to be seeded with at least 10 weight percent of SAPO-31 crystals. (See Examples 51 and 53 of Lok et al.). If SAPO-31 seed crystals were not used a detectable portion of the resulting crystalline material was not SAPO-31 but in fact has been attributed to some impurity. (See Example 52 of Lok et al.). Because of the very similar properties of the SAPO-31 molecular sieve and the impurity, purification techniques are not effective in removing the SAPO-31. Accordingly, heretofore, the art was not able to prepare substantially pure SAPO-31 without the addition of significant amounts of SAPO-31 seed crystals.